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Figure 2.8: AEM depth slices 0 – 5 m, 5 – 10 m, 10 – 15 m and 15 – 20 m (colour stretch 0 – 0.5 S/m)

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Figure 2.9: AEM depth slices 20 – 30 m, 30 – 40 m, 40 – 60 m and 60 – 80 m (colour stretch 0 – 0.5 S/m)

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Figure 2.10: AEM depth slices 0 – 5 m, 5 – 10 m, 10 – 15 m and 15 – 20 m (colour stretch 0 – 1.2 S/m)

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Figure 2.11: AEM depth slices 20 – 30m, 30 – 40m, 40 – 60m and 60 – 80m (colour stretch 0 – 1.2 S/m)

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Figure 2.12: AEM depth slices 80 – 100m and 100 – 150m (colour stretch 0 – 1.2 S/m)

2.4 Basement features revealed by the AEM The AEM maps three curvilinear resistive features in the far south-west of the flight area. These

features trend north-west to north-north-west (parallel to the regional strike of the Lachlan Fold Belt)

and first become evident in the 20 to 30 m depth slice. Drilling has revealed these are basement

structures – sub-cropping ridges of Hervey Group Sandstone.

The survey also maps the subsurface extension of the prominent exposed topographic ridge of Hervey

Group Sandstone 15 kilometres south-west of Narromine. The deeper AEM slices (greater than 80

metres) show the fault bounding the topographic ridge continues to the north-north-west, and because

this fault appears to have caused an offset in the overlying Mesozoic rocks, it must have been re-

activated during or after deposition of the Mesozoic sediments.

The AEM also provides new information on the subsurface morphology of the Mt. Foster – Mt. Harris

monzonite complex. Several resistive monzonite pinnacles in addition to the known outcrops are

revealed buried beneath conductive Rolling Downs Group sediments at various depths (Figure 2.13).

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Figure 2.13: Extent of buried monzonite pinnacles revealed by the AEM (0 – 0.5 S/m colour stretch) in the Mt Foster – Mt Harris intrusive complex

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2.5 Uniqueness of the AEM bulk conductivities It is important to point out that bulk conductivity should only be considered to be constant in the

saturated zone. In the unsaturated zone, bulk conductivity varies according to antecedent soil moisture

(Mullen and Kellett 2004). When antecedent soil moisture is low, bulk conductivity declines; when

soil moisture is high, bulk conductivity rises and attains a maximum when the soil reaches field

capacity. The variations in bulk conductivity because of fluctuations in soil moisture are greatest in the

0 to 5 metre depth slice, and to a lesser degree, in the 5 to 10 metre depth slice. The effect is greatest in

irrigation areas and around rivers, where riparian vegetation (particularly river red gums) may exhaust

the soil water supply and tap into shallow groundwater during drought.

The lower Macquarie River Valley AEM was flown during drought when antecedent soil moisture

was at an historic low. Also, irrigation allocations had been severely reduced since 2001, therefore

bulk conductivities in irrigation areas would be low becaue of the reduced soil moisture. This means

that the bulk conductivities shown in the 0 to 5 metre slice should be considered as minimum values,

particularly in the irrigation areas and riparian zones.

0

200

400

600

800

1000

1200

1400

1600

1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

95th percentile

5th percentile

2006: 270 mm (survey flown late in year)

Figure 2.14 Narromine annual rainfall (mm) since 1888. The AEM survey was flown during drought when antecedent soil moisture was at an all-time low and irrigation had been substantially reduced since 2001

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